1. Field of the Invention
The present invention relates to a liquid developer used for electrophotographic image forming apparatuses such as copying machine, printer, digital printing machine, and the like.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a document image or an image based on image data is exposed on an electrostatic-latent-image carrier such as photoreceptor to thereby form an electrostatic latent image, the electrostatic latent image is developed with toner into a visible toner image, and the toner image is transferred to and fixed on a recording material to thereby form an intended image.
The development scheme used for such electrophotography may be classified into a dry development method and a wet development method. The dry development method uses only toner particles in performing development. In contrast, the wet development method uses a liquid developer (also called wet developer) in which toner particles are dispersed in an electrically insulating liquid (simply referred to as “insulating liquid”, such an insulating liquid is also called carrier liquid) in performing development. The wet development method can use toner particles of a smaller particle size than that of the dry development method, and therefore can obtain a high definition image.
For example, while the limit of the particle size of toner particles for the dry development method is on the order of 5 μm, the particle size of toner particles for the wet development method can be reduced to a size on the order of submicron. Moreover, regarding the wet development method, reduction of the amount of consumed toner particles can be expected.
A preferred insulating liquid used for the liquid developer of the wet development method has a resistance to the extent that will not disturb the electrostatic latent image (on the order of 1011 to 1016 Ω·cm). The insulating liquid is more preferably a solvent without odor and toxicity. In general, examples of such an insulating liquid may be aliphatic hydrocarbon, alicyclic hydrocarbon, aromatic hydrocarbon, halogenated hydrocarbon, polysiloxane, and the like. In particular, in terms of odor, harmlessness, and cost, straight-chain or branched aliphatic saturated hydrocarbon (normal paraffin-based solvent or isoparaffin-based solvent) is suitably used.
For example, Japanese Laid-Open Patent Publication No. 2007-041162 discloses EXAMPLEs in which a first liquid paraffin (aliphatic saturated hydrocarbon) having a weight-average molecular weight of 250 (corresponding to a carbon number of 18) and a second liquid paraffin having a weight-average molecular weight of 800 (corresponding to a carbon number of 57) are mixed into an insulating liquid of a liquid developer, and discloses a result that the fixing strength is excellent. For this fixing, however, a condition is employed that an oven is used to perform thermal fixing at 120° C. for 30 minutes. Since the energy used here is extremely large, the fixing performed here is not an actually available one. Thus, the actually available fixing energy does not enable this insulating liquid to be sufficiently vaporized, and therefore it is expected that the fixing strength is weaker.
Japanese National Patent Publication No. 07-502604 discloses EXAMPLEs in which an insulating liquid of a liquid developer contains Isopar G (carbon number 10: 44%, carbon number 11-12: 56%), which is a commercially available product of aliphatic hydrocarbon, and a small amount of mineral oil (with a carbon number of 18 or more) added thereto. This insulating liquid is a combination of Isopar G having very high volatility and mineral oil having very low volatility. If the mineral oil is absent or the amount of added mineral oil is extremely small, offset occurs. If a greater amount of mineral oil is added, the fixing strength is weaker. Thus, prevention of offset and improvement of the fixing strength cannot both be achieved.